Neural time course of echo suppression in humans.

In reverberant environments, the brain can suppress echoes so that auditory perception is dominated by the primary or leading sounds. Echo suppression comprises at least two distinct phenomena whose neural bases are unknown: spatial translocation of an echo toward the primary sound, and object capture to combine echo and primary sounds into a single event. In an electroencephalography study, we presented subjects with primary-echo (leading-lagging) click pairs in virtual acoustic space, with interclick delay at the individual's 50% suppression threshold. On each trial, subjects reported both click location (one or both hemifields) and the number of clicks they heard (one or two). Thus, the threshold stimulus led to two common percepts: Suppressed and Not Suppressed. On some trials, a subset of subjects reported an intermediate percept, in which two clicks were perceived in the same hemifield as the leading click, providing a dissociation between spatial translocation and object capture. We conducted time-frequency and event-related potential analyses to examine the time course of the neural mechanisms mediating echo suppression. Enhanced gamma band phase synchronization (peaking at approximately 40 Hz) specific to successful echo suppression was evident from 20 to 60 ms after stimulus onset. N1 latency provided a categorical neural marker of spatial translocation, whereas N1 amplitude still reflected the physical presence of a second (lagging) click. These results provide evidence that (1) echo suppression begins early, at the latest when the acoustic signal first reaches cortex, and (2) the brain spatially translocates a perceived echo before the primary sound captures it.

Pattern of BOLD signal in auditory cortex relates acoustic response to perceptual streaming.

BACKGROUND: Segregating auditory scenes into distinct objects or streams is one of our brain's greatest perceptual challenges. Streaming has classically been studied with bistable sound stimuli, perceived alternately as a single group or two separate groups. Throughout the last decade different methodologies have yielded inconsistent evidence about the role of auditory cortex in the maintenance of streams. In particular, studies using functional magnetic resonance imaging (fMRI) have been unable to show persistent activity within auditory cortex (AC) that distinguishes between perceptual states. RESULTS: We use bistable stimuli, an explicit perceptual categorization task, and a focused region of interest (ROI) analysis to demonstrate an effect of perceptual state within AC. We find that AC has more activity when listeners perceive the split percept rather than the grouped percept. In addition, within this ROI the pattern of acoustic response across voxels is significantly correlated with the pattern of perceptual modulation. In a whole-brain exploratory test, we corroborate previous work showing an effect of perceptual state in the intraparietal sulcus. CONCLUSIONS: Our results show that the maintenance of auditory streams is reflected in AC activity, directly relating sound responses to perception, and that perceptual state is further represented in multiple, higher level cortical regions.

Auditory attentional control and selection during cocktail party listening.

In realistic auditory environments, people rely on both attentional control and attentional selection to extract intelligible signals from a cluttered background. We used functional magnetic resonance imaging to examine auditory attention to natural speech under such high processing-load conditions. Participants attended to a single talker in a group of 3, identified by the target talker's pitch or spatial location. A catch-trial design allowed us to distinguish activity due to top-down control of attention versus attentional selection of bottom-up information in both the spatial and spectral (pitch) feature domains. For attentional control, we found a left-dominant fronto-parietal network with a bias toward spatial processing in dorsal precentral sulcus and superior parietal lobule, and a bias toward pitch in inferior frontal gyrus. During selection of the talker, attention modulated activity in left intraparietal sulcus when using talker location and in bilateral but right-dominant superior temporal sulcus when using talker pitch. We argue that these networks represent the sources and targets of selective attention in rich auditory environments.

Auditory grouping mechanisms reflect a sound's relative position in a sequence.

The human brain uses acoustic cues to decompose complex auditory scenes into its components. For instance to improve communication, a listener can select an individual "stream," such as a talker in a crowded room, based on cues such as pitch or location. Despite numerous investigations into auditory streaming, few have demonstrated clear correlates of perception; instead, in many studies perception covaries with changes in physical stimulus properties (e.g., frequency separation). In the current report, we employ a classic ABA streaming paradigm and human electroencephalography (EEG) to disentangle the individual contributions of stimulus properties from changes in auditory perception. We find that changes in perceptual state-that is the perception of one versus two auditory streams with physically identical stimuli-and changes in physical stimulus properties are reflected independently in the event-related potential (ERP) during overlapping time windows. These findings emphasize the necessity of controlling for stimulus properties when studying perceptual effects of streaming. Furthermore, the independence of the perceptual effect from stimulus properties suggests the neural correlates of streaming reflect a tone's relative position within a larger sequence (1st, 2nd, 3rd) rather than its acoustics. By clarifying the role of stimulus attributes along with perceptual changes, this study helps explain precisely how the brain is able to distinguish a sound source of interest in an auditory scene.